If President Obama wants a firsthand look at how politics makes life difficult for immigrants, he ought to stop by a New York City carwash.

The cliché holds that immigrants do the dirty work Americans aren’t willing to do. In our carwashes, the cliché is reality.

According to the Association of Car Wash Owners, Gotham is home to 118 carwashes. Most are individually owned, and most owners are immigrant entrepreneurs.

In addition, virtually all the men and women who work at New York’s carwashes, about 3,000 in all, are also immigrants.

Asked if the workers and owners come from any particular country or ethnicity, one proprietor says: You name them, we’ve got ’em — everyone from Haitians and Pakistanis to Chinese and Chileans. “We are the United Nations of business,” he says.

Right now the carwashes have become the target of a well-funded and -organized campaign to paint them as bad employers who exploit their workers. The goal of this campaign — which includes a bill in the City Council — is to give the industry no option but to unionize.

Now, let’s stipulate that shining hubcaps or vacuuming car interiors for $6.05/hour plus tips may not be most people’s idea of the American Dream.

Even so, the carwashes raise a question far too often ignored in politics: the near-irreplaceable role played by small, unglamorous, low-wage businesses in a city.

For millions of New Yorkers, these businesses provide important services at prices they can afford. For the workers who toil in them, they provide a vehicle for upward mobility.

Think of it this way: For a 22-year-old guy who has just arrived here from Guatemala, doesn’t speak English and hasn’t got much in the way of an education, the reality is that wiping a windshield may be the best option he’s got, at least at the start.

Small wonder so many of this city’s unfashionable jobs — taxi driver, housekeeper, food vendor, etc. — are done by immigrants.

And yet these same jobs are crucial for any city that hopes to keep its lower-skilled residents out of poverty.

Jason Riley helps explain why in his new book “Please Stop Helping Us.” Riley notes that for black households in poverty, the problem isn’t a worker who isn’t earning enough. It’s that no one in the house has a job.

“Having a job gives people training they can’t get anywhere else,” says Peter Cove of America Works, an innovative national organization dedicated to lifting people out of poverty by getting them into the workplace and helping them stay there.

“It teaches you how to relate to people in an appropriate way, and it teaches you the real skills employers are looking for.”

In “My Fair Lady,” Eliza Doolittle agrees to let Henry Higgins teach her “proper English” so she can move from selling flowers in the street to working in a nice shop. Likewise in New York, a carwash worker might realize the way to a bigger paycheck is to become a manager.

The progressive answer to helping such workers is to unionize and then force owners to raise pay — without any thought to the tradeoffs. But in an industry where the basic price for a full-service carwash goes for about $11, the tradeoffs are human.

If the costs go too high, the owners will simply eliminate the labor-intensive interior cleaning and go back to back to what carwashes used to be in the old days, when all you did was clean the exteriors by running them through the tunnel wash.

So what do you do about bad bosses? Well, if a carwash owner is breaking the law or stealing tips from his workers, he should be prosecuted. But a growing economy that is constantly throwing up new jobs is probably more important.

Because the best guarantee of good treatment is when a worker is in a position to tell her boss to stuff it — knowing she can easily get another job somewhere else.

We don’t hear much from Gotham’s political class about this kind of job protection, the security that comes from opportunity. At least in New York, the praise and press releases (not to mention the millions in tax credits) for businesses are generally reserved for the glam industries creating the “good” jobs.

Ask yourself this: When was the last time you heard a pol celebrate a dry cleaner or bodega?

So maybe we’d do better by workers if we changed our thinking about enterprises such as carwashes — and what it means to work in one. Peter Cove suggests we start by getting rid of the whole notion of “dirty” jobs.

“Any job gives you dignity,” he says. “If you take it, you do your best and you use it to move up the economic ladder, it’s not really a dirty job, is it?”